Pseudomonas aeruginosa is one of the chief causative agents of infection in severely granulocytopenic patients with acute leukemia and other malignancies. The prime objectives of this study has been to monitor its epidemiology in cancer patients and its natural history in the hospital environs. Recovery rates from blood cultures are higher than last year. P. caryophylic from food was probably the original source of its incidence in the upper respiratory tract of five patients and in several denture cups. P. cepacia was isolated from denture cups in three patients at a later time. A strain of P. stutzeri with an unusual cross-resistance to penicillins was isolated from Micro-Bact, the toilets and sink sprays in the LAF units.